|In Every Colour, At Every Price.
If there were
awards given to gemstones for their various attributes and qualities,
then the top prize for versatility would surely go to tourmaline.
Tourmaline occurs in every colour possible from colourless to black
and every shade and hue in between. In fact, there are more than a
hundred shades to choose from when one gets down to it. Sometimes
a single stone will display more than one colour as in the case of
bi-colour and tri-colour tourmalines.
is not just one mineral, it is a whole family of minerals related
in physical and is a complex silicate of boron and aluminium. Many
different elements are able to find a spot for substitution in the
crystal lattice and this accounts for all the different colours of
the gem. The exact species of tourmaline is determined by which of
a number of possible elements is present in the specimen.
Tourmaline forms in a variety of geological settings. Mostly, it is found in granite pegmatites and in their immediate vicinity in the enclosing host rock. Pegmatitic tourmaline is commonly black and is associated with microcline, albite, quartz and muscovite. The light coloured gem tourmalines are much more rare, usually occuring in pegmatite core zones with quartz, clevelandite, muscovite, lepidolite and more rarely amblygonite and spodumene.
Other areas of occurrence for tourmalines are with quartz in hydrothermal veins where heated mineral bearing liquids or gases from deep igneous sources later cooled and crystallised along rock fractures, in granites due to late stage alteration of micas and
feldspars by boron containing fluids, and by boron metasomatism in contact and regionally metamorphosed rocks. Because of tourmaline's relatively high hardness and specific gravity, it is often found in elluvial and alluvial deposits.
As it is a
pegmatite mineral, it is found in the world's great pegmatite districts.
Foremost among these is the land of gems, Brazil. They are also found
in California and Maine in the United States. Apart from the Americas,
there is an abundance of the mineral in Africa as well, mainly in
the East African nations of Kenya, Tanzania, Mozambique and Madagascar.
Malawi produces some beautiful canary yellow material, while gorgeous
rubellites and fine blue-green tourmalines can be sourced in Nigeria.
Nor is Asia left behind as gem material can be found in Afghanistan,
Pakistan, Burma and Sri Lanka as well.
It is composed of
long, prismatic crystals. Tourmaline crystals are often cracked and
flawed, especially the reds, pinks and bi-colours, while the greens
and blues are generally much cleaner.
There is one particular iharacteristic of tourmaline that is of great advantage to gemstone alters. Tourmaline is a strongly pleochroic gem. The darkest colour is always seen looking down the ixis of the crystal. This feature allows gem cutters to play with the gem cutting darker stones in a way that will display the lighter of the two pleochroic colours or conversely, cut a lighter stone in a manner that displays the darker shade. If the gem is cut so that the c axis is oriented parallel to the table facet, then the stone will appear lighter in colour. If it is cut in a manner that orients the table perpendicular to the c axis, then it will display the richer colour in the stone.
There are many other interesting
characteristics to this gem. When faces are present on both ends of
a natural crystal, they can be seen to have developed with different
orientations and do not correspond. This phenomenon is known as hemimorphism,
and it gives the stone certain electrical properties. Tourmaline is
pyroelectric, which means that it develops a charge when it is heated.
The Dutch traders noticed this and used long unfashioned crystals
to draw ash from their meerschaum pipes and thus, they labelled this
gem 'ashentrekker', which literally means ash puller.
Tourmaline also displays piezoelectricity when its crystal is placed under stress. Pressure will lead to the stone's charging and in the course of discharging, the plus and minus poles will alternate so that the stone will show vibrations like rock crystal, only it is much stronger in tourmaline. This property makes it very useful for application in underwater detection equipment, and depth and pressure guages. During World War II, it was much in demand for use in the production of pressure sensitive guages and submarine instruments as well as other war equipment. It has also been used as a calibration standard for the manometer and as a standard to check for the possible effects of water soluble boron used in mixed fertilisers.
Another feature of tourmaline is that when slices of it are cut from a prism face along the length of the crystal, then they have the ability to polarise light.
As mentioned earlier,
tourmaline is a family of groups of minerals rather than a single
Dravite is brown
in colon and is rarely transparent enough to be considered gem material.
However, it can be heat treated to lighten the dark colour and is
therefore sometimes cut into gems. Its crystals are typically elongated
three sided prisms.
Uvite was earlier
thought to be a rare form of tourmaline but it has now become fairly
common in the mineral market. Uvite is the magnesium/iron rich member
of the tourmaline family and has one notable exception to the typical
tourmaline generalised formula, in which the 6 aluminium atoms are
reduced to 5 and a magnesium atom is inserted into the missing aluminium's
are recognisable by their prismatic form, typically elongated like
pencils with cross sections that range between hexagonal and trigonal.
Elbaite is also the most strongly pleochroic of the tourmaline family.
It is the mineral responsible for almost all the gem varieties of
tourmaline. Although elbaite is a separate group of minerals in the
tourmaline family, in the gem trade this name is used to refer to
green tourmaline only. All the other forms of elbaite have their own
is a rare variety of green tourmaline that is found only in Kenya,
Zambia, Tanzania and Namibia. The finest chromes rival the top colour
of emerald and offer a more durable stone for jewellery. The colour
is due to the presence of chromium or vanadium. It was first mined
in Tanzania in the early 1960's.
The most famous and rare of all tourmalines is the paraiba nirmaline. It is one of the world's most unusual gemstones. It was discovered in Brazil in 1982 and at first it was called electric tourmaline and then subsequently neon tourmaline. However both these names lacked the cachet of paraiba (derived from the state of Paraiba in Brazil where it is found), which is the name that stuck.
Paraiba has the most fascinating colour range of any gemstone yet - vivid turquoise, electric blues, neon green, rich twilight blues. It is the only variety of tourmaline that contains copper. A study conducted by the German Foundation for Gemstone Research found that paraiba tourmaline also contains significant amounts of gold, nearly 8.6 parts per million. When you take into account that the average gold content of the earth's crust is 0.007 parts per million, you can appreciate exactly how high the gold content in paraiba tourmaline is. In fact, if it were not such a beautiful, expensive (retail prices are said to go up to $20,000 per carat for the finest specimens) and rare gem, it would be crushed to salvage the gold.
Like tanzanite, paraiba
tourmaline is also found in only one location in the world, in a football
field sized patch of land near the village of Sao Jose da Batalha
in the state of Paraiba in northeastern Brazil. Production is sporadic
and doesn't keep pace with market demand. The mine shafts are hand
excavated tunnels upto 60 metres deep and the gem is found only in
thin veins. Therefore, supply will always be strictly limited and
the prices for this gem are not likely to ever decrease.
Experts theorise that the presence of this unique gem
in only two locations on two separate continents can be explained
by the theory of continental drift. Continental drift is the seperation
of one original super continent into the different continents we know
today by the force of geological processes and if the coastlines of
South America and Africa were put together they would fit each other
just like the pieces of a jigsaw puzzle. Therefore, it is possible
that the gemstone was created in both Brazil and Nigeria under similar
conditions before the two continents drifted apart.
Then there is rubellite
tourmaline, which is named for the deepest reds that look like rubies.
Eye clean rubellite is among the most expensive kinds of tourmaline
because as mentioned earlier, the reds have a tendency to be heavily
,included. There is also a rubellite garnet which is generally clean
material and is also at a fraction of the price of rubellite tourmaline.
There is also a rare alexandrite tourmaline which changes colour just like alexandrite. Its daylight colouring is yellowish/brown green which turns to red under evening light. Chatoyancy is seen in the cat's eye tourmaline and this is due to the crystals growing fluid filled tubes along the long, prismatic axis. These are usually cut as cabochons. There are also orange and yellow varieties of tourmaline that do not have any special name to distinguish them and are known by their colour like pink tourmaline is. Anchorite is a colourless variety of tourmaline. There is also an attractive purplish coloured tourmaline that is called siberite. This name applies only to the stones found in Russia's Ural Mountains.
The Empress Dowager Tz'u Hsi, the last Empress of China, particularly loved pink tourmaline. She is reputed to have bought a ton of this material from the Himalaya Mine in California and she is said to be buried on a carved tourmaline pillow. The good news is that you do not have to be an empress to enjoy this beautiful gem. Apart from the diversity of colour, its availability also swings from easily available at a few dollars a carat to the rare can be had only for thousands of dollars a carat. Therefore there is a tourmaline to suit every taste, every pocket and allows for bragging rights along a very diverse economic spectrum.
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